The invention relates to a method for determining a current temperature of a heating element with a PTC thermistor property (PTC) with which a urea-water solution for cleaning the exhaust gas of an internal combustion engine of nitrogen oxides can be heated.
The invention also relates to a device for determining a current temperature of a heating element with a PTC thermistor property (PTC) which heats the urea-water solution in order to clean the exhaust gas of an internal combustion engine of nitrogen oxides, and wherein a temperature dependence of the resistance of a reference heating element is stored in a control unit.
Exhaust gas purification systems for denitrifying the exhaust gas of an internal combustion engine utilize a urea-water solution which is metered into the exhaust gas upstream of a catalytic convertor for selective catalytic reduction (SCR catalytic convertor). The urea-water solution (“Ad-Blue”) freezes at a temperature of −11° C. and must therefore be heated up under cold ambient conditions. A freezing solution expands and can therefore damage the metering system. Furthermore, the solution cannot be metered and the exhaust gas would contain unacceptably high values of nitrogen oxide. Because of the intrinsic safety of a heating element with a PTC thermistor characteristic (PTC-positive temperature coefficient-heater), such a heating element is used to heat the urea-water solution. The resistance of the heating element rises strongly with the temperature starting from a material-dependent switching temperature, and reduces the heating power in the case of a constant operation voltage, with the result that the switching temperature essentially cannot be exceeded. Depending on the necessary heating power, the heating element can be composed of a plurality of PTC heating elements connected in parallel. Even in the case of a heating element which is not covered by a urea-water solution, the PTC characteristic prevents the heating element heating up too strongly as result of the reduced dissipation of heat. In order to thaw the urea-water solution, a much lower temperature is necessary than the switching temperature of the PTC heating element. It is therefore desirable to bring about regulation and limitation of the temperature of the heating element and of the urea-water solution without an additional temperature sensor.
Document DE 10 2013 108 501 A1 describes methods for manufacturing a conveyor module with an electronic PTC heater, for installation in a liquid tank and for storing a liquid additive, having at least the following steps:
a) defining a maximum electrical power which is made available to the conveyor module,
b) defining a thermal conductivity of the conveyor module of a location of the electrical PTC heater in the liquid tank,
c) calculating a switching temperature of the PTC heater from the maximum electrical power and the thermal conductivity, and
d) mounting a PTC material with a corresponding switching temperature for the PTC heater at the location.
In document DE 10 2013 108 501 A1 a working temperature of a PTC heating element, which is reached at a predefined, desired heating power, is determined. A PTC material with a switching temperature which corresponds to the working temperature is selected as the material for the heating element. As a result, the power which is taken up by the heater is limited to the desired value.
Document DE 10 2012 110 985 B4 discloses a method for determining a temperature range for a reducing agent which is located in a liquid tank for an exhaust gas post-treatment system of a motor vehicle with an internal combustion engine, which method comprises the following method steps:
a) sensing a temperature (TT) of the liquid tank,
b) sensing an ambient temperature (TU) of the motor vehicle,
c) sensing the difference between the temperature (TD) of the liquid tank and the ambient temperature of the motor vehicle,
d) defining a temperature (TR) of the reducing agent which is located in the liquid tank, on the basis of the temperature sensed in steps a) to c), and
e) storing the temperatures sensed or defined in the steps a) to d) and
f) repeating the steps a) to e) after a predefined time period.
The temperature of the reducing agent is determined for its optimum metering for exhaust gas post-treatment.
Document DE 10 2010 038 361 A1 discloses a method for measuring the temperature of a medium, wherein the medium is in contact with at least one heating device (18), and the heating device (18) has at least one PTC element, comprising the following method steps:
sensing at least one variable which characterizes the current and/or the voltage and/or the resistance at the PTC element during the operation of the heating device (18),
forming at least one conductance value from the sensed values,
forming at least one predictor (100; 200) from the conductance value which is dependent on time, and
evaluating the at least one predictor (100; 200) in order to infer the temperature of the medium.
In the document it is mentioned that the temperature of the medium surrounding the heating device can be inferred from the conductance value of the heating device. Therefore, the temperature of the heating device can also be inferred from the resistance, said temperature largely corresponding to the temperature of the surrounding medium.
Heating elements with a PTC characteristic are, however, subject to fabrication tolerances, as result of which the relationship of their resistance and their temperature is also subject to tolerances.